Is there a lock or release button? What keeps the saddle in place?Updated a month ago

The main thing that keeps the Dual-Position seatpost locked in either position is your body weight. In order to move between positions, the linkage has to move upward first, so as long as your weight is resting on the saddle, the linkage cannot change positions. There is also a strong spring detent that holds the linkage firmly in position if you're out of the saddle.